By way of general background, in bipolar transistors two types of current carriers, electrons and “holes,” are involved in the conduction mechanism of the device. A bipolar integrated circuit typically comprises many interconnected bipolar transistors, each of which may be a three-terminal device having a base region, a collector region and an emitter region. The collector and emitter regions of a transistor may be doped with impurities that are of the same conductivity type, for example n-type or p-type, and the base region may be of the opposite conductivity type. The transistor is then to be of the n-p-n or the p-n-p type.
Growth in both high-frequency wired and wireless markets has introduced new opportunities where semiconductors such as SiGe have unique advantages over bulk complementary metal oxide semiconductor technology. Because of the characteristics of the bipolar transistor, and because of the signal processed by the transistor in the radio frequency circuit is a signal having high frequency, the bipolar transistor, especially the SiGe heterojunction bipolar transistor is frequently employed in the radio frequency circuit.